Ceramic materials have properties which make them of interest in many applications. These properties include thermal stress resistance, oxidation and corrosion resistance, and high strength. Selected cuprate perovskites have been recently identified as superconductors with high critical temperatures [Chu, C.W., et al., Phys. Rev. Lett. Jpn. J. Appl. Phys. 26, L1 (1987)]. Two silicon based ceramic materials, silicon carbide (SiC) and silicon nitride (Si.sub.3 N.sub.4), have found use as engineering materials in high temperature applications because of their high melting temperatures, low coefficients of thermal expansion, and high resistance to oxidation. However, they are strongly covalently bonded and dissociate rather than melt. These properties make them difficult to sinter to full density. Further, they are highly abrasive and thus difficult to machine.
Methods for the formation of silicon based ceramic forms have included hot-pressing, reactive bonding and sintering with or without additives. Each of these processes has advantages, be they in cost or in the properties of the resulting material, however, the adhesion of the ceramic surface layer has been suspect in most applications of date. There have been no known references to date describing use of laser-energy-induced surface reactions to form silicon based ceramic layers on surfaces which layers resist peeling on thermal cycling or nominal physical abrasion.
Methods for formation of selected cuprate perovskites as superconductors with high critical temperatures include metathetic processes involving heating, pressing and sensitization in air or oxygen atmospheres of mixtures of copper oxide with materials such as a barium oxide, barium carbonate or barium nitrate and yttrium oxide. Additional preparation methods include sputtering of these elements, electron beam evaporation, melt-processing, and eximer-laser based film formation by preformed surface ablating via evaporation and material transfer to a receiving surface. There have been no known references to date of a laser based method of formation of a cuprate perovskite at a metal surface using laser based energy in such a manner so that the cuprate perovskite remains superconducting with a high critical temperature and is linked to the metal surface so that the two materials (e.g., the cuprate perovskite and the metal) do not separate under repeated temperature cycling or nominal physical abrasion.
Laser processing of metal surfaces has been successfully used to modify metal surface properties either by forming in situ a protective metal surface coat (i.e., stellite or tribaloy on steel) or by resolidifying the underlying metal. Techniques have been developed using a high power laser where metal powder layers are applied and melted in continuous processes to provide protective layers or regions on metal surfaces from the metal powders without reaction between the metal powder and the metal surface. The resulting protective layers have significantly improved erosion or corrosion resistance [Brennan, E.M. and B.H. Kerr, Chapt. in Laser Materials Processing, M. Bass, Ed., North-Holland, NY. p.237 (1983)]. For example, layering of conventional stainless steel (AISI 304) with a laser formed metal alloy containing 29% chromium and 13% nickel significantly improved its corrosion resistance [Lurnsden, J.B., et al., Chapt. in Corrosion of Metals Processed by Directed Energy Beams, C. R. Clayton and C.M. Preece, Eds., AIME, Warrendale, PA, p 129 (1982)]. The resulting metal surface layer has a chemical composition which differs from the bulk metal but the surface is not a ceramic in that it is not made essentially from a nonmetallic mineral.
Laser processing parameters for surface modification must be tailored to obtain the desired extent of surface melting or layer formation. Parameters of importance include laser wavelength, power density, time of exposure (target velocity, laser pulse duration and frequency), and inert gas coverage of sample. The required values of these parameters Will vary both with substrate and layering materials.
Laser processing of a surface involves surface layer heating due to carrier scattering and phonon formation of the absorbed incident light. Absorption of the laser energy into a metal, carbon or ceramic surface can cause the localized temperature to rise sufficiently to cause melting and surface recrystallization with extent and depth dependent upon thermophysical properties of the substrate surface. Melt depth, solidification velocity and cooling rate affect the properties of the processed surface. By way of example, with the example not intended to limit the type of laser which can be used in the subject invention, a Nd:YAG (Raytheon SS-501) laser rated at 400 watts with a pulse width of 2 milliseconds was used to heat metal surfaces. Surface temperatures of metals heated with this laser are on the order of 2000 to 3000.degree. C. given a measured bulk temperature of 600.degree. C. and an estimated solidification velocity of approximately 10 cm/sec and a cooling rate of 10.sup.6 .degree.K/sec. At temperatures on this order of magnitude, surface metal atoms will undergo significant electronic excitation (e.g., radical formation) Which in turn means that they may chemically react with active gaseous species at the metal-gas interface. Such reactions should produce a surface layer which incorporates underlying surface atoms chemically linked to the silicon, carbon, copper, nitrogen or other reactive atoms in the ceramic. Whether there is a negative gradient of atoms from the bulk substrate surface to the ceramic layer surface or whether the ceramic layer surface has an excess of bulk substrate atoms due to density differences of the molecules formed in the laser based process will affect the properties of the ceramic layer.